Fluorochemicals (e.g., fluorinated and perfluorinated organic compounds) are commercially valuable and useful chemical materials. Fluorochemicals can exhibit various useful properties, e.g., they may be inert, nonpolar, hydrophobic, oleophobic, etc. As such, fluorochemicals can be useful in a wide variety of applications. They can be useful as oil, water, and stain resistant chemicals; they can be useful as refrigerants and heat exchange agents; or as solvents and cleaning agents. Due to the versatility of fluorochemicals, and a consequent strong demand for these materials, there is a continuing need in the fluorochemical industry for new and improved methods of preparing fluorochemicals.
One well-known industrial process for preparing fluorochemical compounds is the electrochemical fluorination process commercialized initially in the 1950's by the 3M Company. This process, often referred to as Simons fluorination or electrochemical fluorination (ECF), is a method by which electric current is passed through an electrolyte solution containing a mixture of liquid anhydrous hydrogen fluoride and an organic compound intended to be fluorinated (the "substrate"). Generally it is taught that the Simons process is practiced with a constant current passed through the electrolyte; i.e., a constant voltage and constant current flow. See for example W. V. Childs, et al., Anodic Fluorination in Organic Electrochemistry, H. Lund and M. Baiser eds., Marcel Dekker Inc., New York, 1991. The current passing through the electrolyte causes one or more of the hydrogens of the substrate to be replaced by fluorine.
The Simons process of electrochemical fluorination, although commercially useful, includes aspects that might desirably be improved upon. For example, the Simons process requires a significant amount of electrical energy passing through the electrolyte solution. Much electrical energy is effectively used to fluorinate the substrate, but a certain amount of this electrical energy converts to heat energy that must necessarily be carried away from the electrochemical fluorination cell as wasted energy, and adds to the overall cost of operating the process. It would be desirable to reduce the amount of electrical energy that is wasted as dissipated heat energy in the Simons process, and thereby reduce the overall cost of electricity needed to operate this process.
Also, the conventional Simons process often includes the use of conductivity additives to allow the passage of current through the electrolyte solution. See for example J. Burdon and J. C. Tatlow, The Electrochemical Process for the Synthesis of Fluoro- Organic Compounds, Advances in Fluorine Chemistry, edited by M. Stacey et al., volume 1 p. 129 (1960). Conductivity additives can cause undesired results when used in the Simons process. Conductivity additives, for example, can interfere with the fluorination of the substrate, either by causing increased corrosion of the anode, or by themselves being consumed or fluorinated in the fluorination reaction. This can reduce the overall yield of the desired fluorinated product, and in many ways can increase the costs of the fluorination operation. Therefore, it would be desirable to reduce or even substantially eliminate the need for conductivity additives.
Finally, the Simons process can be difficult to maintain at steady state for extended periods of time because high resistance by-product films and tars can tend to accumulate on electrodes of the fluorination cell, specifically, at the anode. In normal operation, the accumulation of films and tars on the anode causes increased resistance of the electrochemical cell, and an upward drift in cell voltage. The problem can become more serious and lead to the condition referred to as "current blocking," which is manifested as a permanent increase of resistance and loss of conductivity within the cell. To correct current blocking often requires shut-down of the apparatus for cleaning. It would therefore be desirable to prevent increases in the resistance of a fluorination cell that can lead to loss of conductivity within the cell, and the permanent condition of current blocking.